Abstract

Chlorofluorocarbon (CFC), sulfur hexafluoride
(SF6), tritium (3H), and tritium/helium-3 (3h/3he) concentrations
in discharge from wells and springs in karst or fractured
rock of the Blue Ridge and Valley and Ridge of parts of
Virginia and Pennsylvania indicate that ages and mixing
fractions can be interpreted using a binary mixing model
in which a young component (apparent age of 0-25 yrs) is
diluted to varying extents with old, pre-tracer water. In
contrast, SF6 and 3h/3he data for water from shallow springs
that discharge from residuum overlying fractured crystalline
rock of the Blue Ridge can be interpreted using either exponential
or piston-flow models (mean ages 0-3 yrs). Use of multiple
tracers and "tracer plots" can eliminate some
mixing models and refine estimates of mean tracer age. About
half of the 3h/3he samples (39 total) from the Valley and
Ridge karst have initial tritium concentrations consistent
with piston flow (0-15 yrs, unmixed). The rest of the samples
are shown to be mixtures of young and old (pre-bomb) water.
The CFC data also demonstrate cases of piston flow and binary
mixing in the Valley and Ridge, but can be affected by contamination.
Other samples demonstrate apparent removal of CFC-11 and
CFC-113, and excesses of SF6 and 4He from terrigenic sources.
The tracer data show that most ground water from fractured
and/or karstic aquifers is highly susceptible to contamination.
Use of Halon 1211 in air rotary drilling shows that the
effects of drilling can leave a long-lasting imprint on
"natural" environmental tracer concentrations.